Efficient Homogeneous Hydridoirida‐β‐Diketone‐Catalyzed Methanolysis of Ammonia‐Borane for Hydrogen Release in Air. Mechanistic Insights

The hydridoirida‐β‐diketone [(IrH{(PPh 2 ( o ‐C 6 H 4 CO)) 2 H}) 2 (μ‐Cl)][BF 4 ] ( 2 ) has been used as a homogeneous catalyst for the methanolysis of ammonia‐borane to release up to 3 equivalents of hydrogen in the presence of air. With catalyst loadings as low as 0.2 mol%, ammonia‐borane undergoes methanolysis within 6 min at 30 °C, with TOF 50% of 320 mol H2  ⋅ mol Ir −1  ⋅ min −1 , or within 80 s at 60 °C, with an excellent TOF 50% of 1991 mol H2  ⋅ mol Ir −1  ⋅ min −1 , and maintains its catalytic activity in consecutive runs. Triethylamine‐borane fails to undergo methanolysis. Kinetic studies indicate first‐order dependence on substrate and on catalyst concentration and suggest cleavage of the solvent O−H bond being involved in the rate determining step of the reaction. In methanol solution 2 forms cationic [IrH(MeOH){(PPh 2 ( o ‐C 6 H 4 CO)) 2 H}] + ( 3 ) and reacts with Me 3 N−BH 3 to afford a hydridoirida‐β‐diketone [IrH(Me 3 NBH 3 ){(PPh 2 ( o ‐C 6 H 4 CO)) 2 H}] + ( 4 ), with the borane adduct η 1 ‐coordinated to iridium. Compound [ 4 ][BAr F 4 ] shows dynamic behaviour in solution due to exchange of bridging and terminal B−H bonds. A multinuclear NMR study of the catalyzed reaction shows the formation of two ammonia‐methoxyborane adduct intermediates, H 3 N−BH 2 (OCH 3 ) and H 3 N−BH(OCH 3 ) 2 , and an iridium species proposed of the hydridodiacyl type [IrH(H 3 NBH 3−x (OCH 3 ) x )(PPh 2 ( o ‐C 6 H 4 CO)) 2 ] with a coordinated borane adduct. On account of experimental evidence, a simplified catalytic cycle is suggested for the methanolysis of AB to release hydrogen.